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Secular Variation (secular + variation)
Selected AbstractsSecular variation of hemispheric phase differences in the solar cycleASTRONOMISCHE NACHRICHTEN, Issue 8 2010N.V. Zolotova Abstract We investigate the phase difference of the sunspot cycles in the two hemispheres and compare it with the latitudinal sunspot distribution. If the north-south phase difference exhibits a long-term tendency, it should not be regarded as a stochastic phenomenon. We use datasets of historical sunspot records and drawings made by Staudacher, Hamilton, Gimingham, Carrington, Spörer, and Greenwich observers, as well as the sunspot activity during the Maunder minimum reconstructed by Ribes and Nesme-Ribes. We employ cross-recurrence plots to analyse north-south phase differences. We show that during the last 300 years, the persistence of phase-leading in one of the hemispheres exhibits a secular variation. Changes from one hemisphere to the other leading in phase were registered near 1928 and 1968 as well as two historical ones near 1783 and 1875. A long-term anticorrelation between the hemispheric phase differences in the sunspot cycles and the latitudinal distribution of sunspots was traced since 1750 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstructionGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Henry C. Halls A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China's largest dyke swarm, which trends for about 1000 km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U,Pb dating of primary zircon, is 1769.1 ± 2.5 Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D = 36°, I = , 5°, k = 63, ,95 = 4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp = 2°, dm = 4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800 Ma [source] Convection in the Earth's core driven by lateral variations in the core,mantle boundary heat fluxGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2000Steven John Gibbons Summary Moving core fluid maintains an isothermal core,mantle boundary (CMB), so lateral variations in the CMB heat flow result from mantle convection. Such variations will drive thermal winds, even if the top of the core is stably stratified. These flows may contribute to the magnetic secular variation and are investigated here using a simple, non-magnetic numerical model of the core. The results depend on the equatorial symmetry of the boundary heat flux variation. Large-scale equatorially symmetric (ES) heat flux variations at the outer surface of a rapidly rotating spherical shell drive deeply penetrating flows that are strongly suppressed in stratified fluid. Smaller-scale ES heat flux variations drive flows less dominated by rotation and so less inhibited by stratification. Equatorially anti-symmetric flux variations drive flows an order of magnitude less energetic than those driven by ES patterns but, due to the nature of the Coriolis force, are less suppressed by stratification. The response of the rotating core fluid to a general CMB heat flow pattern will then depend strongly on the subadiabatic temperature profile. Imposing a lateral heat flux variation linearly related to a model of seismic tomography in the lowermost mantle drives flow in a density stratified fluid that reproduces some features found in flows inverted from geomagnetic data. [source] Secular variation of hemispheric phase differences in the solar cycleASTRONOMISCHE NACHRICHTEN, Issue 8 2010N.V. Zolotova Abstract We investigate the phase difference of the sunspot cycles in the two hemispheres and compare it with the latitudinal sunspot distribution. If the north-south phase difference exhibits a long-term tendency, it should not be regarded as a stochastic phenomenon. We use datasets of historical sunspot records and drawings made by Staudacher, Hamilton, Gimingham, Carrington, Spörer, and Greenwich observers, as well as the sunspot activity during the Maunder minimum reconstructed by Ribes and Nesme-Ribes. We employ cross-recurrence plots to analyse north-south phase differences. We show that during the last 300 years, the persistence of phase-leading in one of the hemispheres exhibits a secular variation. Changes from one hemisphere to the other leading in phase were registered near 1928 and 1968 as well as two historical ones near 1783 and 1875. A long-term anticorrelation between the hemispheric phase differences in the sunspot cycles and the latitudinal distribution of sunspots was traced since 1750 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Retrieving geomagnetic secular variations from lava flows: evidence from Mounts Arso, Etna and Vesuvius (southern Italy)GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Alberto Incoronato Summary Mean directions of magnetization from Mounts Arso (Ischia Island, Gulf of Naples), Etna and Vesuvius lava flows have been determined based on very stringent linearity criteria. These indicate that, regardless of the source volcano, the lava flow mean directions of magnetization form a common path, the SISVC (Southern Italy Secular Variation Curve). This curve enables a reassessment of the age of eruption of several lavas. A date of AD 1169 is demonstrated to be the only possible time of emplacement for one Etna lava flow previously assigned an age of AD 812/1169. It is also demonstrated that two Etna lava flows, which, according to the literature, were emplaced in AD 1536 and 1595 respectively, were actually both emplaced around AD 1037. Three other Etna lava flows, one ascribed to AD 1566 and two to AD 1595, were actually emplaced between AD 1169 and 1284/85. The same time window also holds for a Vesuvius lava flow for which only an upper time threshold was previously available. Only one of the studied flows needs further sampling and analysis to verify whether this flow has been affected by a complete remagnetization or has an erroneous historical dating. The applied procedure seems to be the most appropriate one in carrying out palaeomagnetic surveys of lava flows, as also suggested by the broad agreement with some 17th and 19th century measurements of the geomagnetic field in Rome, relocated to Etna, and is likely to improve knowledge of past history of a volcano significantly. [source] Breastfeeding Rates in Hong Kong: A Comparison of the 1987 and 1997 Birth CohortsBIRTH, Issue 3 2002Gabriel M. Leung MD ABSTRACT: Background: Low breastfeeding rates are an issue of international public health concern. Anecdotal reports suggest very low breastfeeding rates in Asia, but no population-based studies have been conducted in the region. To determine the secular trend in breastfeeding practice in an Asian postindustrialized metropolitan community, we examined data from two population-based birth cohorts of Hong Kong infants in 1987 and 1997. Methods: Annual population rates of breastfeeding initiation and duration were estimated from the birth cohorts, considering the change in breastfeeding rates over 10 years with correction for sociodemographic and birth characteristics. Factors associated with breastfeeding practice were identified using multivariate logistic regression modeling in a pooled analysis of individual data of both cohorts. Results: Overall, 26.8 percent of mothers initiated breastfeeding in 1987, and the rate increased to 33.5 percent in 1997. The rate would have been 27.4 percent in 1987 if the distributions of method of delivery, birthweight, birth order, maternal age, education, and employment status had been the same as in 1997. Only 7.6 percent of infants remained on the breast for more than 1 month in 1987 compared with 20.4 percent a decade later. Similarly, the rate for breastfeeding more than 3 months increased from 3.9 to 10.3 percent. Total breastfeeding duration was significantly longer in 1997 than 10 years earlier. Conclusions: This is the first systematic report of secular variations of breastfeeding rates in Asia. Hong Kong should set higher but realistic goals for breastfeeding that emphasize both initiation and maintenance. Given the wide latitude for improvement in terms of readily modifiable risk factors, such as smoking and cesarean section, these new goals should focus on improving rates in these targeted groups where breastfeeding rates are lowest. (BIRTH 29:3 September 2002) [source] Late Weichselian pollen stratigraphy, clay varve chronology and palaeomagnetic secular variations in Lake Bolmen, Småland, south SwedenBOREAS, Issue 3 2001JONAS ISING Pollen analysis, glacial varve chronology and palaeomagnetic measurements were carried out on Late Weichselian lake sediments from southwestern Smaland, south Sweden. The sequence is correlated to the GRIP event stratigraphy, expressed in calendar years BP, and covers the period from the deglaciation at c. 14 400 to 11 300 calendar years BP. The series encompasses c. 930 varves and has been connected to the local varve chronology. Varve thickness increases markedly after the Older Dryas stadial, which indicates an accelerated deglaciation and melting of dead ice. The pollen diagram displays the vegetation development from the deglaciation at c. 14 400 calendar years BP to the transition to the Holocene. The vegetation succession starts with an arctic pioneer vegetation at the deglaciation, changes to a more stable tundra environment and displays a development which concurs with the traditional lateglacial pollen stratigraphy for southern Sweden. A palaeo-magnetic secular variation curve is presented displaying two westerly declination swings at 14200-13800 and 12 800-11 600 calendar years BP, respectively. The upper one can be recognized from other palaeomagnetic stratigraphies from southern Sweden and Estonia. [source] | ||||||||||